1. Field of the Invention
The invention relates to cordierite ceramic bodies for use as catalyst carriers, particularly to cordierite bodies of high strength, low thermal expansion and high porosity for use as catalyst carriers for purifying automobile exhaust gas, and a method for producing the cordierite bodies.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NO.sub.x) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of gaseous emissions from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970's.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to high thermal shock resistance of cordierite ceramics. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion. That is, honeycombs with a low thermal expansion have a good thermal shock resistance and can survive the wide temperature fluctuations that are encountered the application.
The production of cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) ceramics from mineral batches containing sources of magnesium, alumina and silica such as clay and talc is well known. Such processes are described in U.S. Pat. No. 2,684,919. The manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low expansion coefficients along, at least one axis, is disclosed in U.S. Pat. No. 3,885,977.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to optimize the thermal shock resistance and strength of the cordierite substrates. The following patents each relate to the manufacture of ceramic honeycombs exhibiting improved thermal shock resistance or coefficient of thermal expansion (CTE) and/or strength.
U.S. Pat. No. 5,144,643 (Beall et al.) discloses the method of fabricating a cordierite body having at having 12 to 16%, by weight MgO, 35-41% Al.sub.2 O.sub.3 and 43-53% SiO.sub.2. Furthermore, the body comprises least 90%, by weight, cordierite and a coefficient of thermal expansion less than about 9.times.10-.sup.7 /.degree. C. from about 25 to about 1000.degree. C. The method involves selecting specific raw materials that will form the desired cordierite body. Specifically, these raw material selections should not include any clay or talc, should include a MgO-yielding component and an Al.sub.2 O.sub.3 -yielding component having a particle size of no greater than 15 and 8 micrometers, respectively. The raw materials are mixed together, subsequently dried and fired for a time and a temperature sufficient to form the aforementioned cordierite body. Given the low CTE and the degree of microcracking likely at these CTE values, the cordierite bodies disclosed herein are relatively weak.
U.S. Pat. No. 5,332,703 (Hickman) discloses a ceramic product consisting essentially of cordierite and the method for making the same. Specifically, the ceramic body is comprised of at least 90%, by weight, cordierite, exhibits an MOR of at least 3500 psi, a total porosity not exceeding about 20%, by volume, and a CTE not exceeding about 4.8.times.10-.sup.7 /.degree. C. from about RT to about 800.degree. C. The method for producing the aforementioned ceramic articles involves employing a combination batch comprising a mineral component and a chemical component. The mineral batch component comprises clay and talc while the chemical component consists essentially of powdered oxides, hydroxides, or hydrous oxides of magnesium, aluminum and silicon. The raw material batch combination is mixed, formed, dried and thereafter fired to result in the formation of the ceramic body. Although the Hickman reference exhibits the desired combination of low CTE and relatively high strength it does so at the expense of a reduction in the porosity. This porosity tradeoff is unacceptable, given that a high porosity is important in the production of honeycomb catalyst carrier substrates, specifically it is needed for the proper application of high surface area washcoats.
While such ceramics represent an improvement in properties over extruded cordierite ceramics produced using more conventional processes, still further improvements in these products, particularly with respect to strength, without a reduction in porosity and an unacceptable increase in thermal expansion would be desirable. Strength has become an increasingly important consideration in production of cordierite honeycomb substrates as a result of the move to producing thinner-walled, higher cell density, increased catalytic conversion efficiency and lower back pressure-producing cordierite honeycombs catalyst carriers. Put differently, there is a move towards increasing the intrinsic material strength of the honeycomb substrates in order to compensate for the loss in strength resulting from the move toward decreasing the honeycomb substrate web strength.
It is therefore a principal object of the present invention to provide improved cordierite ceramics, and method for making them, such that the ceramics exhibit higher strength in combination with inherent low thermal expansion and high overall porosity.